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\begin{equation*} \rho \cdot \left({\partial w \over \partial t}+ u {\partial w \over \partial x}+ v {\partial w \over \partial y}+ w {\partial w \over \partial z}\right) = k_z -{\partial p \over \partial z} + {\partial \over \partial z} \left[ \mu \cdot \left(2 \cdot {\partial w \over \partial z}-\frac{2}{3}\cdot (\nabla \cdot \mathbf{v}) \right) \right] + {\partial \over \partial x}\left[\mu \cdot \left({\partial w \over \partial x} + {\partial u \over \partial z}  \right) \right] + {\partial \over \partial y}\left[\mu \cdot \left({\partial v \over \partial z} + {\partial w \over \partial y}  \right) \right]\end{equation*}
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